feat(nixos): improve nix configuration and tmp handling

- Reformatted function parameters to enhance readability.
- Added `build-dir = "/var/tmp"` to nix settings for better build management.
- Removed redundant `TMPDIR` environment variable for nix-daemon in systemd services.

modules/nixos/services/base/default.nix

@@ -1,7 +1,8 @@
-{ config
-, lib
-, pkgs
-, ...
+{
+  config,
+  lib,
+  pkgs,
+  ...
 }:
 with lib;
 with lib.metacfg;
@@ -188,7 +189,6 @@ in
     virtualisation.libvirtd.enable = lib.mkDefault true;
 
     boot.tmp.useTmpfs = true;
-    systemd.services.nix-daemon.environment.TMPDIR = "/var/tmp";
 
     services.fstrim.enable = true;
   };